In recent years, there has been an ever-increasing demand for power electronics to drive LED based lighting systems. Along with this is the demand for improving the power quality of the AC systems supplying the power electronics. Various options are available to improve the power quality of AC systems. One of the options that is gaining in popularity is the active shaping of the input line current (i.e., active Power Factor Correction (PFC)), since it makes the load appear as a pure resistance, leading to near unity load power factor and generating negligible harmonics in the input current.
One well-known active PFC LED driver is a single-ended primary inductor converter (SEPIC). This type of driver uses only one switch to harmonize the input current and to regulate the current through the LEDs. Even more preferable, is to use the SEPIC converter operating in discontinuous conduction mode (DCM). This is a cost effective and simple solution because operating in DCM allows the converter to operate as a voltage follower, where the input current naturally follows the input voltage profile without the use of an inner current control loop. Operating in DCM additionally reduces switching losses because switch turn-on occurs with zero current, and the output diode does not present reverse recovery current.
For potentially harsh environments, such as those encountered by aerospace exterior light systems, isolation of load from the input is desired from product reliability perspective. Thus, an isolated SEPIC is even more preferable for these types of environments. However, because of the insulation voltage requirements associated with such environments, close coupling between primary and secondary windings of the isolation transformer is not allowed. As a result, both the primary and secondary transformer windings exhibit considerable leakage inductances. This can lead to undesirably large voltage spikes during switching operations. These large voltage spikes can reduce overall circuit reliability, and can generate undesirable electromagnetic interference (EMI).
Hence, there is a need for an isolated SEPIC that does not generate undesirably large voltage spikes during switching operations and/or that provides improved circuit reliability and/or that does not generate undesirable electromagnetic interference (EMI). The present invention addresses at least these needs.